oxygen sensor readings

[GRUMPYNOMO]

Don't think of the O2 sensors indications as rich or lean.  Think of them as
"all oxygen used up" (around 900mv) and "some oxygen left over after
combustion" (around 100mv).
This may seem picky but if you see a low voltage and assume the engine is
running lean, you may be looking at incomplete combustion or a miss.
More significant yet is a jagged waveform at the high (rich?) side which might
look like a cylinder that is lean but could be 'puff' of EGR gas (not rich)
from a faulty valve.
Sensors vary but popular pattern is a variation from 0mv to 100mv at the low
side to  800mv to 900mv at the high side with an average of 450mv.
Oxygen sensors will never stay at the mid point.  They will flip-flop from
seeing no spare oxygen to seeing some oxygen remaining in exhaust stream.
They are incredible devices: so rapid and sensitive that, with a good scope,
you can identify vacuum leaks and bad injectors.  They are a very
underutilized diagnostic tool.